In sodium based pulp mills the black liquor containing spent chemicals from the digestion of cellulosic vegetable fibres and in some cases the spent liquor from an oxygen bleaching/delignification stage is passed to a chemical recovery system where the black liquor is first reduced in volume by evaporation. Thereafter the concentrated material is incinerated, for example, in a furnace or by means of a fluidised bed reactor. Finally the incinerator product, which in soda processes contains a high percentage of sodium carbonate, is dissolved in water and causticised to convert the sodium carbonate to sodium hydroxide and the "white liquor" so obtained is returned for re-use in the digestion process.
Although soda processes exclude the use of sulphur containing chemicals for the digestion of wood and while oxygen bleaching of the pulp excludes the use of chlorine, both of these contaminants may be introduced into the process as constituents of the raw materials and process water used. Potassium compounds are also contained in wood and of course where sea water floated logs are concerned substantial quantities of chlorides are introduced into the process. In a closed pulping and recovery circuit, a build-up of contaminants such as chlorides, sulphates and potassium compounds may therefore occur.
When a fluidised bed reactor is used to incinerate concentrated liquor containing these contaminants, loss of fluidisation of the reactor bed will occur if the concentration of the contaminants exceeds a critical level.